(Quantum tunneling and High harmonic generation)
Physicists are
perfectly aware that the microscopic behavior of electrons cannot be understood
without the laws of quantum theory. When scientists trace the dynamics of
subatomic phenomena, they like to ask questions that are motivated by a classical,
non-quantum perspective. In this spirit, researchers determine the exact time
the electron exits the atoms that are irradiated by a short flash of laser
light. The existence of such an exit time is seemingly counter intuitive, given
that electrons are described by wavefunctions
that extend smoothly from the inside to the outside of
atoms, some part of the electron is always outside the atom.
The emission of
electrons in Shafir and Colleagues’ experiment is a consequence of quantum
tunneling. The applied laser field changes the potential energy profile,
experienced by the electrons, forming a finite barrier, which classical
Newtonian particles are not able to penetrate, but which can be tunneled across by electrons. A similar process forms the basis of scanning
tunneling microscopy: electrons tunnel between surface of objects and the tip
of the microscope. Tunneling occurs because electron wavefunctions encompass
both sides of a potential barrier, so what is the meaning of an exit time?
Shafir and colleagues’
report suggests that high- harmonic emissions from helium atoms are described
by ‘quantum orbits’. This means that tunneling proceeds in imaginary time ( the
imaginary part of time as defined by a complex number), but electron moves as a
classical particle in ‘real’ time, once it has excited the atom. At the start
of its real time journey, the electron counter intuitively moves toward the
parent ion.
The superposition of
the many different associated electron trajectories form a quantum mechanical
wave packet- a short ‘pulse’ of travelling wave activity- for emitted electrons.
The result of Shafir and colleagues’ experiment is in excellent agreement to
the ‘quantum orbit model’. Just the imaginary part of time changes for the
electron as it tunnels through a potential barrier; time becomes real
valued-only at the exit of the tunnel. This real time is the exit time measured
by Shafir and colleagues’. It is the time electrons start to feel the effect of
the probe- field.
By facilitating the real
time observations of attosecond electron dynamics, this approach will increasingly
compete with ultrafast spectroscopic techniques in which molecules are directly
probed by attosecond light pulses.
(Ref. Shafir et al., Nature. vol. 485, 2012)
